This research plan will seek to further the understanding of the function of Vif(virion infectivity factor) encoded by human immunodeficiency virus type I (HIV-1). Vif is essential for viral replication either in vivo or in culture for non-permissive cells, such as peripheral blood lymphoid cells, macrophages and H9 T-cells. Defects in the vif gene will affect the virion morphology, but not the expression of viral components. Data from our laboratory and others have indicated that Vif affects reverse transcription, possibly because of the instability of reverse transcription (RT) complex in specific target cells. As Vif protein is not specifically packaged into virions, it is believed that Vif functions in the virus- producing cells and affects viral assembly. Recently, we demonstrated that Vif is an RNA binding protein and binds to HIV-1 genomic RNA in the cytoplasm of virus-producing cells to form a 40 S messenger ribonucleoprotein (mRNP) complex. The coating of Vif to viral RNA may protect the viral RNA from various endogenous inhibitors in the cytoplasm. in addition, Vif in mRNP complexes may mediate viral RNA engagement with HIV-1 Gag precursors during formation of a budding particle. As such, the formation of a proper nucleocapsid complex could be altered by vif gene defect. With a long term goal of understanding the molecular mechanism(s) underlying this unique lentiviral protein, we will perform the following series of experiments to study the function of Vif: (l) Dissect the structure and components of RT complexes of vif- viruses. (2). Further define Vif-RNA interactions and the role of Vif in the process of RNA packaging; (3). Identify cellular proteins which interact with Vif (Vif Cofactor). Thus, these focused and interrelating experiments will allow precise analyses of the molecular mechanism(s) of Vif protein in the assembly of RT machinery, and determine the effects of vif gene on HIV-1 pathogenesis in vivo.